This invention relates to laboratory apparatus useful in the assay of biological and biochemical reactants and is particularly concerned with multi-well filtration devices capable of retaining liquids for substantial periods of time prior to performing filtration.
Test plates for in vitro analysis which contain a multiplicity of individual wells or reaction chambers are commonly known laboratory tools. Such devices have been employed for a wide variety of purposes and assays as exemplified by U.S. Pat. Nos. 3,694,464; 4,304,865; 4,276,048; 4,154,795; 4,427,415; 4,526,690 and Re. 30,562. Microporous membrane filters and filtration devices containing such microporous membranes have become especially useful with many of the recently developed cell and tissue culture techniques and assays, particularly those in the field of virology and immunology. Typically a 96-well filtration plate is used to conduct multiple assays simultaneously some steps of which last several hours prior to performing filtration. With such filtration plates, especially those containing microporous membranes, there is a well recognized and recurrent problem in that liquid in the wells tends to pass through the membrane by capillary action and gravity flow thereby causing a loss of contents from within the reaction well. Before the desired stage in the experimental design, prevention of liquid loss by capillary action and gravity flow becomes especially important when living cells or tissues are being maintained or grown within the reaction wells. Under these circumstances, favorable media conditions for the cells or tissues must be maintained for hours or even days and any loss of liquid from the wells, however small, will affect the condition of the cells and influence the results of the assay. Prevention of liquid loss through the membrane in this manner is also vitally important when the assay utilizes very small sample volumes as reactant, such test samples often being less than 100 microliters in volume. The pendant drop that invariably forms on the underside of the membrane due to such capillary action and gravity flow is typically about 50 microliters in volume and it is apparent that a liquid loss of such proportions must drastically affect the assay.
In many instances, the test design involves a step of recovering the liquid from the well so the liquid can be analyzed. A common problem with prior art apparatus is that drops from adjacent wells often merge during the filtration step thereby cross-contaminating the samples. In addition, large pendant drops may remain attached to the underside of the membrane structure thereby giving variable sample transfer. Furthermore, in some prior art devices, porous hydrophobic surfaces are utilized and air passing through the hydrophobic areas can disturb and scatter the pendant drops so that liquid from the well is not recovered in the designated recovery well. Accordingly, it would be desirable to provide a multi-well filtration plate which permits conducting tests having varying incubation times and which permits recovery of liquid separately from each well so the liquid can be analyzed, if desired.